The Biomarker Distributions of Light Oils and Condensates in the Xihu Depression, East China Sea Basin

Abstract

Summary The Xihu Depression in the East China Sea Basin is a petroliferous depression. Recently, many breakthroughs continuously have been made it one of the most promising regions for petroleum exploration in the East China Sea. However, the origin of the petroleum in the Xihu Depression still remains controversial and is not well understood. In order to investigate the thermal maturity, depositional environment, and source materials of light oils and condensates, gas chromatography-mass spectrometry analyses of the saturated fractions were performed. The distribution of n-alkanes, C29 sterane isomerization ratio and C31 22S/(22S+22R) homohopane ratio indicates that the light oils and condensates were generated within the early to early peak oil window. The dominance of C29 regular steranes and the almost complete absence of C27 regular steranes, the abundant diterpanes and the presence of oleanane suggest a dominant contribution from terrigenous source materials. The high Pr/Ph ratios, dominance of C29 steranes, distribution of hopanes, near-absence of gammacerane indicate that the source rocks for the hydrocarbons were deposited in a relatively oxic environment.